There is a tubular fuse as a safety device for protecting an electric circuit from an excess current. As shown in FIG. 11, a generally-used tubular fuse 1 is made up of a glass tube 2, a fuse element 3 arranged in the glass tube 2 along an axial line of the glass tube 2, and a pair of cap terminals 4 (only one side is shown) fitted on both ends of the glass tube 2. Both ends of the fuse element 3 protrude and are soldered to the outside of the glass tube 2 through element insertion holes 4a formed to the respective cap terminals 4. This causes the fuse element 3 to be directly supported in the center of the glass tube 2 by each cap terminal 4 with a predetermined gap between the fuse element 3 and the glass tube 2. It is to be noted that both end portions 3a (indicated by a two-dot long and two short dashes line in the drawing) of the fuse element 3 protruding from solder 5 after soldering are shaved off by using a knife or the like.
In this tubular fuse 1, however, the solder 5 is piled up on the surface of the cap terminal 4 to fix the cap terminal 4 and the fuse element 3 protruding from the element insertion hole 4a, which makes it difficult to automate the process and leads to the deteriorated productivity. That is, in case of external soldering, an advanced technique is required for soldering for maintaining the quality of the fuse. For example, if the solder is overflowed from the bottom to the outer peripheral surface of the cap, this can cause an imperfect contact with the fuse holder. Further, in order to obtain the sufficient strength, the soldering must be applied on the entire bottom surface of the cap around the element insertion hole 4a of the cap terminal 4 without making any unsoldered part. Furthermore, when the soldering is not finished in one time and a soldering iron is again put to cover the unsoldered or insufficient part, the hardened solder may be melted to loosen the fuse element 3. Only a skilled operator can, therefore, carry out the soldering and the automation is not realized so far. However, the soldering causes the poor working environment that may cause a flux or the like to be generated, leading to the shortage of operators.
In addition, since the soldering is so applied as to cover the element insertion hole 4a of the cap terminal 4, the melted solder may bring the air therein and a cavity may be generated inside the solder 5 and above the element insertion hole 4a in particular, thereby preventing a desired strength from being obtained. Detection of this cavity is hard by the visual test, and hence it does not give a serious question to fusion of the fuse caused by the usual excess current if such a cavity exists. However, if a short-circuit or the like occurs near the fuse and a large current flows to cause interruption, the element may explode to blow the solder into pieces and to cause jet of an arc flame or gas or emission of metal powder.
Moreover, since the soldering is performed with the glass tube 2 being erected in such a manner that the solder 5 is piled up from the top, the melted solder may run down from the element insertion hole 4a before being hardened to wrap the fuse element 3 therein, and the length of the exposed part of the fuse element 3, i.e., the fuse length may become unequal to give the irregularity to the pre-arcing time/current characteristic.
Additionally, as a tubular fuse enabling the automation of manufacture, there is one shown in FIG. 12. In this tubular fuse 6, both end portions of the fuse element 3 are externally bent along the edges of the glass tube 2 and sandwiched between the glass tube 2 and a cap terminal 7 to be fixed. According to this fuse structure, the fuse element 3 can be temporarily fixed by only fitting the cap terminal 7 and, if the solder 5 is piled up inside the cap terminal 7 in advance, the soldering can be effected by only heating in this state to fix the fuse element 3. Here, since the cap terminal 7 must be fitted with both the ends of the fuse element 3 being caught on the edges of the glass tube 2 and being stretched, the both ends are bent in the direction opposed to each other in the diameter direction. The fuse element 3 is, therefore, obliquely housed in the glass tube 2.
In this tubular fuse 6, however, the fuse element 3 directly contacts with and is obliquely supported by the glass tube 2, and hence the fuse element 3 may be brought into contact with the glass tube 2 in the vicinity of the edges of the glass tube. When the fuse element 3 is brought into contact with the glass tube 2, the heat of the fuse element 3 may be transferred to the glass tube 2 and fusion of the fuse element 3 may be delayed.
Further, since the solder 5 is filled and directly fixed between the inner surface of the cap terminal 7 and the end surface of the glass tube 2, the inside of the glass tube 2 is sealed to disable ventilation when the cap terminal 7 is heated in order to melt the solder 5 provided on the inner surface of the cap terminal 7 and to fix the solder 5 to the fuse element 3, the cap terminal 7 and the glass tube 2. An increase in the pressure inside the glass tube 2 causes application of the melted solder to be uneven so that the ununiformity be generated at a position where the solder 5 is attached to the fuse element 3, and the length of the exposed part of the fuse element 3 thereby becomes unequal to give the irregularity to the pre-arcing time/current characteristic. Also, in some cases, the solder 5 can not be attached and fixed to the fuse element 3 on the edges of the glass tube 2, and the fuse element 3 is in direct contact with the glass tube 2, i.e., the fuse element 3 can not be supported by the solder, which may damage the bent portions of the fuse element 3 or cause disconnection due to fatigue of a metal. Since the inside of the glass tube 2 is sealed, an increase in the inner pressure owing to explosion of the element can not be released at the time of interruption caused due to a short-circuit accident or the like, and the glass tube 2 may explode or the cap terminal 7 may be blown out. The strength of the glass tube 2 or the joining force of the cap 7 must be considerably increased.
Furthermore, since the cap terminal 7 is fitted with the both ends of the fuse element 3 being stretched in the opposed directions along the diameter direction on the edges of the glass tube 2 in order to sandwich the both ends between the glass tube 2 and the cap terminal 7, the relatively-large cap terminal 7 is inclined to the glass tube 2 and can not be concentrically provided with respect to the glass tube 2. In particular, this tendency becomes prominent as the wire diameter of the fuse element 3 increases. The positions of the cap terminals 7 provided on both sides are, therefore shifted with respect to the glass tube 2 in the opposed directions along the diameter direction by the thickness of the fuse element 3. As shown in FIG. 13, when the tubular fuse 6 is fitted to fuse clips, the point contact causes each cap terminal 7 to be heated and softens the hard solder 5 to loosen the stretched fuse element 3, and the fuse element 3 is brought into contact with the glass tube 2, which leads to an imperfect fusion or the melted fuse clips.
Moreover, when mounting the tubular fuse 1 shown in FIG. 11 or the tubular fuse 6 shown in FIG. 12 on, e.g., a printed wiring board as a fuse with a lead wire, such a fuse may be supplied to a solder tank of a reflow furnace or the like and thereafter soldered on the printed wiring board. In this case, if the temperature management of the reflow furnace or the like is poor, the solder layer 5 fixing the fuse element 3 becomes soft when the tubular fuse is heated, the stretched fuse element 3 may be loosened to be brought into contact with the glass tube 2.
It is, therefore, an object of the present invention to facilitate automation of the process and provide a reliable fuse.